1. Field of the Invention
The present invention relates to a knock detection device for an internal combustion engine and, more particularly, to a device for detecting knock based on an ion current inside an combustion chamber.
2. Description of the Related Art
In a gasoline engine, the air/fuel mixture in the vicinity of a spark plug is ignited by the spark produced at the spark plug, and gasoline combustion takes place with the ignited flame propagating throughout the entire air/fuel mixture. Knocking is one kind of abnormal combustion. It is a phenomenon in which the air/fuel mixture self-ignites before the flame front arrives when there occurs an abnormally rapid rise in pressure during flame propagation. When knock occurs, combustion gases oscillate, which allows heat to propagate more freely, and in some cases, engine damage may result. Knocking is closely related to ignition timing; as the ignition timing is advanced, maximum combustion compression increases, increasing the tendency to knock.
On the other hand, it is desirable to increase the compression ratio in order to increase thermal efficiency and reduce fuel consumption. To achieve this, it is practiced, as part of ignition timing control, to advance the ignition timing up to the limit where knock is about to occur by detecting the occurrence of knock. Previously, in this kind of knock detection method, it was common practice to detect knock-induced vibrations using a vibration sensor attached to the cylinder block or like part, but in recent years, a knock detection method has been proposed that utilizes the change of an ion current inside a cylinder when knock occurs.
More specifically, when a spark is produced at the spark plug and air/fuel mixture burns in the combustion chamber, the air/fuel mixture is ionized. When the mixture is in the ionized state, if a voltage is applied to the spark plug, an ion current flows. The occurrence of knock can be detected by detecting and analyzing this ion current. Usually, when knock occurs, an oscillating component of 6 kHz to 7 kHz appears in the ion current. The knock detection device based on the ion current extracts this frequency component peculiar to knock by means of a filter, and judges the knocking condition based on the magnitude of that component.
As an example, Japanese Unexamined Patent Publication No. 61-57830 discloses a device which detects knock by extracting a particular frequency component associated with knock from an ion current signal. The construction of this device is such that a capacitor as an ion current generating source is charged to a given voltage by the secondary current produced when the primary current of the ignition coil is shut off and, after spark discharge, the ion current is measured that flows through a closed circuit consisting of the capacitor, the secondary winding of the ignition coil, the spark plug, and a current detecting resistor.
However, when the secondary winding of the ignition coil (the secondary coil) is placed in the ion current flow path, as in the above prior art, an LC resonant circuit is formed by its inductance L and the stray capacitance C associated with the coil and spark plug. As a result, when rapid changes occur in the ion current, LC resonance occurs within the ion current path and a resonant current flows.
The resonance frequency of such a resonant current is determined by the values of the inductance L and capacitance C within the ion current path. If this resonance frequency is close to the knock-induced frequency, a situation can occur where the frequency component for knock detection appears in the ion current despite nonoccurrence of knock. In that case, an erroneous decision is made that knock has occurred when actually knock has not occurred.